O-1 Re-Heat Treat

Thanks for reminding me of another way to harden just the edge of a blade. The thought behind my responce to Kevin was based on it didn't click in my mind that when one did a soft draw on the spine of a blade they were starting out with tempered martinsite and changing it to a softer temper and that the martinsite, even with the soft draw, was still stronger than the pearlite that would result from differential hardening. My comment about heating the spine with a torch until it was austinised to convert the tempered martinsite to pearlite was more to illustrate the impracticallity of doing so.

Doug Lester
 
Man this place is great! Thank you gentlemen for such an exchange of information. Kevin, I’d love to be able to follow you around for a while and just listen. I almost always learn something every time I read one of your posts.

I have always used carbon steels, 5160 and 1084, and done a partial quench because I believed it produced a superior knife. Now, after reading your posts, I am forced to re- evaluate my thinking. This is especially true since I generally make hunter and smaller ‘user’ types of knives that are not generally subjected to the rigors of larger heavy duty blades.

Would I be correct in thinking that, besides the strength vs toughness issue, a full quench would offer somewhat less margin for error than an edge quench?

Thanks,
Carey
 
When you ask what makes a superior knife blade you have to ask superior for what. Just about everything in knife making is a trade off between opposing traits. Increasing the edge holding ability of a blade decreases the sharpenability. Softening the spine of a blade by drawing the temper or differential hardening may make a tougher blade but it will also make it weaker. A weaker blade may stand up to the stresses of chopping or limited prying better but will be more likely to bend. A stronger blade may resist deformation better but it is more likely to break if bent too far. A hollow ground blade may slice better but be more likely to chip out on the edge. A convex grind my stand up to chopping better but not be quite as good slicing things. There are just trade off after trade off in knife making and they all support different funtions.

Doug Lester
 
What Doug said is spot on, everything in making a blade is a series of carefully balanced compromises. I tend to say “optimize specific properties” and avoid using the word “superior” whenever I can in order to leave the hyperbole for the hype-mongers and attention hounds this business is rife with.

Optimum performance comes from designing the blade and heat treatment specifically for the task the knife is meant for. In an attempt to make Ronco supermatic ginsu type products to match the tall tales needed to sell them, we have kind of dumbed down our knives, which only worked because we pulled off the greater feat of also dumbing down our clients with the marketing. Of course when I say “we” I am not talking about those present in this discussion, but I think you know some of they guys I am talking about.:3:

But real performance comes from looking at a fine slicing blade and making the edge thin, strong and very abrasion resistant while ignoring the guys who say it needs to bend 90 degrees the be the “ultimate knife”. This optimum performance is the opposite of what you want in a large chopper where you would beef the edge geometry up a little bit and switch priorities from abrasion resistance to impact toughness and strength. But oddly enough even this type of blade gains nothing from being able to easily bend. Bendy steel is nothing to stake your life on. Polybius and others document how those who faced the Romans with swords that bent were summarily slaughtered. When the chips are down a bent blade is no more useful than a broken one.

Determine the task for the knife, then focus on the properties that will perform that task best. That task could be chopping, slicing or a general use knife. The chopping knife will need to sacrifice some of the properties of the slicer in order to gain in others and vice versa, while the general using knife that is expected to do it all will have to compromise on all the properties in order to do everything just O.K. If you go with this specialization approach the tricky part is educating your customers enough to deprogram them from the marketing, so they aren’t trying to chop through an iron bar with a fine skinner because of something they saw in a magazine.:rolleyes:
 
Carey Quinn said:
...Would I be correct in thinking that, besides the strength vs toughness issue, a full quench would offer somewhat less margin for error than an edge quench?

Thanks,
Carey
Click to expand...

Carey I would love to tackle this question but I am not certain if I understand it? Are you asking if there could be more to go wrong in a full quench or a differential quench? The best way to hedge your bet for complete transformation, good homogeneous structure and condition is to get the steel cooled as evenly and effectively as possible. Thus a full quench done properly will have less to go wrong than differentially hardening. If I have to do a differential hardening I prefer using clay in order to still be able to get the whole thin under the surface of the quenchant and maximize my transformation.
 
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